Multiplex telecommunication system



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56 OVERALL TRANSMISSION CHARACTERISTIC E UNCOMPENSATED I AUTO CARRIER I I 0 25 400 1000 2000 3000 4000 5000 3 IFREQUENC Y -CYCLES PER SECOND T l QONIPENSATIUN NETWORK CHARACTERISTIC M a I 1': g i

61 I l l l l I 0 25 400 1000 2000 I 3000 I000 5000 FREQUENCY CYCLES PER SECOND I OVERALL TRANSMISSION CHARACTERISTIC COMPENSATED E CARRIER NO.1 CARRIER No.2

M fig l l I I I I I 0 25 400 1M0 2000 3000 4000 5000 FREQUENCY -CYCLES PER SECOND I INVENTOR- Williaflgfi3%fl% BY I ,I ,Z I I I I I I a 2:: M ATTORNEY.

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MULTIPLE-X TELECOMMUNICATION SYSTEM 2 Sheets-Sheet 2 Original Filed Feb. 26, 1936 H I ':.J

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FILTER TRANSLATOR 92 FILTER TRANSLATOR B B -o 90 mm: TRANSLATOR c c "O INVENTOR. wbllzawjiffiimefiz BY 4 g (fr ATTORNEY.

Patented Jan. 31, 1939 MULTIPLEX TELECOMMUNICATION SYSTEM William G. H. Finch, New York, N. Y. Original application February 26, 1936, Serial No.

65,869, now Patent No.

2,047,863, dated July 14, 1936. Divided and this application June 24, G

1936, Serial No. 86,902

4 Claims. (01. 179-2) My invention relates to telecommunications systems and more particularly relates to novel apparatus for and methods of multiplex carrier transmission of picture and other signals to reof my application, Serial No. 65,869, filed February 26, 1936, which matured into Patent No. 2,047,863 on July 14, 1936. l

' The widest use which telepicture systems to-day enJoy is in connection with newspaper service to th at almost any place for the transmission 01' pictures. Inasmuch as the tele-' phone company, however, does not permit any interconnection by a subscriber to its lines, it has been necessary to find some other than physical connection means for transmitting picture signals over the telephone lines. It has been proposed that this might be accomplished by 25 acoustic coupling to the telephone line.

telephone microphone.

30 Although the acoustic method of inducing picture signals into the telephone line has still been proposed as recently as within the past few years, it has many inherent defects which render it impractical. The more serious of the many draw- 35 backs of such an arrangement are the distorted frequency response characteristic of the usual loud speaker, and the even more distorted frequency response characteristic of the carbon microphone ordinarily used in telephone systems.

Although for the transmision of speech, poor frequency characteristics are not material, as the ear tends to correct for audible distortions, they are serious in connection with telepicture systems where distortion would be visually evident on the 4 received picture.

I have discovered a novel electric coupling to the telephone lines which has a linear frequency characteristic with respect to the signals transmitted thereby. Specifically, my system contem- 5 plates 9. novel method of inductively inducing signals into the telephone line at the transmitter by a novel portable member which may be applied to any telephone subscriber station box.

The transmission of low frequencies, for example frequencies below cycles, is greatly attenumote points over telephone lines, and is a division I ated in present telephone systems. Communication requiring signals below 25 cycles may accordingly not be directly transmitted thereby. By modulating an audio frequency tained by demodulation at the receiver. The electrical inducing means of my present invention provides for the linear transmission and recep- 10 tion of such carrier signals. In telegraphy, printing telegraphy and picture transmission systems, such audio frequency carrier operation is preferable.

By establishing a linear wide frequency band 15 transmission characteristic, for example, linear from 25 cycles to 5000 cycles over a telephone line, it is possible to transmit two independent telecommunications signals utilizing two independent audio frequency carrier waves. taneous speech and telegraph or printing telegraph and telepicture transmission-over a single telephone channel is accordingly practical without physical connection thereto, according to my present invention.

Among the advantages other than the well known economic advantages of utilizing a telephone system for telecommunications which my novel coupling means makes available, is the utilization of a portable transmitter. A newspaper reporter, for example, provided with my novel coupling arrangement, may be provided with a portable transmitter which he can employ at any telephone station for transmitting to his main ofllcea picture which has immediate news value. In doing so, he will in no way interfere with the normal operations of the telephone line other than to utilize it as any subscriber when he lifts thev phone of! the hook. v

In a further modification of my invention, I 40 contemplate transmitting the signals induced into the telephone line to the nearest of a number oi strategically placed radio transmitting stations for the purpose of either simultaneous or subsequent transmission of the picture signals from such station. This arrangement would not only reduce the telephone costs but would also provide quicker service for national coverage.

In accordance with my present invention, I contemplate utilizing the wide audio frequency channel for simultaneous transmission of several signals with corresponding independent carrier waves. For example, the telepicture transmission or the picture to the main oflice may be simultaneously accompanied by telegraphy or printing 06 Simulwhich are combined to select telegraphy signals over the same lines but with an independent carrier frequency. Another modiflcation resides in transmitting two independent pictures over the same channel, using separate independent carrier signals.

In accordance with still another modification of my present invention, I contemplate a high speed printing telegraph system employing a plurality of independent tones which are simultaneously transmitted and which correspond to, for example, a Baudot code combination for each character of the printing telegraph system. Corresponding filters at the receiving station are used to segregate the respective constant frequency tone inpulses to actuate the printing telegraph receiver.

It is accordingly an object of my invention to provide novel methods of and means for electrically coupling to public telephone systems for the transmission and reception of signals between remote points.

Another object of my invention is to provide novel methods of a means for transmitting pictures over a telephone system.

A further object of my invention novel methods of and means for transmitting telecommunications signals across telephone lines, by modulating an audio frequency carrier wave and electrically inducing nals into a telephone system.

is to provide Still a further object of my invention is to provide novel methods of and means for transmitting pictures over a telephone line utilizing an audio frequency carrier wave which is electrically induced into the telephone system.

Another object of my present invention is to provide novel methods of and means for electrically coupling a telepicture receiver at a telephone receiving station for receiving signals without physical interconnection with the telephone system.

Another object of my invention is to linearly transmit two or more independent or interdependent telecommunications signals over a single telephone line without physical connection thereto A further object or my invention is to simultaneously transmit picture and printing telegraphy signals over a single telephone line without physical interconnection thereto.

Still a further object of my invention is to provide a high speed printing telegraph system employing a plurality of independent tone impulses the printing characters, over a telephone line without physical interconnection thereto.

These and other objects of my invention will become apparent in the following description taken in connection with the drawings, in which:

Figure 1 is a schematic circuit diagram of a single channel telecommunication system corresponding to Figure 2 of the parent application, Serial No. 65,869, referred to above.

Figure 2 is an over-all transmission characteristic cur-ve plotted with the ordinate as the voltage at the receiving station translator as received from a constant voltage signal transmitted over the frequency range, the transmission being uncompensated.

Figure 3 is a transmission characteristic of a compensation network necessary to compensate tliie characteristic of Figure 2 for linear transmiss on.

Figure 4 is an over-all transmission characteristhe resulting sigtic between the transmitter and translator units of a compensated system.

Figure 5 is a schematic circuit diagram of a duplex intercommunication system employing two independent modulated carrier signals.

Figure 6 is a schematic illustration of a modifiedmultiplex carrier wave communication system'over a telephone line without physical con-' nection thereto.

The audio frequency signals are inductively coupled into a public telephone system. The telephone system illustrated has a transmitter station It, having a microphone transmitter II and ear-piece l2, connected by leads l3, l4 and IE to the station phone box It, operatively connected to the receiver telephone station l1, having its carbon microphone l8 and ear-piece l9 which are connected by leads 20, 2| and 22 to the receiving station call-box 23, by the interstation leads 24 and 25.

I have here illustrated one of the several methods in which the induction coils 26 and 21 are generally connected in telephone systems. The particular connections of the induction coils in the system are not important,.since in every case the effects described in conjunction with the illustrated connections will be efiected. The dashed 24'-25' of the telephone line 24--25 apparatus necessary to maintain and H in operative relation. tions l0 and 11, as is well known, stations, batteries, ringers. etc.

At the transmitter station It, H is connected to line 25 by lead iii. The microphone H circuit is completed by lead M to the line 24, with the primary 28 of induction coil 26 connected in .series therebetween. As is well known, the carbon microphone requires a direct current energization which is supplied through the last mentioned series circuit arrangement. The telephone earpiece I2 is coupled as follows to the telephone lines to eliminate the the microphone phone I2 is connected to the secondary 29 of the induction coil 26 by lead l3. The opposite side of the secondary 29 is connected to line 25 in series with the condenser 30.

The primary 3! and secondary 32 of the receiver induction coil 21', together with condenser 33 are connected at the receiver microphone l8 and earpiece IS in a manner similar to that at the transmitter l0 herein described.

, The audio frequency signal generator 34 generates the signals to be transmitted over the telephone system lll-l1. Generator 34 may,-ior example, produce picture signals to be transimtted, telegraphy signals or the like. An audio frequency oscillator 35 is operatively connected with the audio signal generator 34, producing at the output 35-41 of generator 34, a signal modulated carrier wave. The signal modulated carrier is amplified by audio frequency amplifier 38 and connected to 4|--42. A compensation network 43 is shown connected between the solenoid and the signal amplifier 38, the purpose of which will be described hereinafter.

The transmitter solenoid 40 is placed adjacent to the induction coil 25 to provide for optimum induction of the telecommunications signals therethe solenoid 40 by output leads to one end of induction coil 26 to insure maximum signal induction. The phone box IE is enclosed by, a sheet metal cover 39 indicated in dotted lines. In a practical performance of the coupling arrangement according to my invention, therefore, one side of the solenoid 40 is juxtaposed with the metal cover 39 at a region so that its axis coincides with the induction coil 26 axis, or is as close thereto as physical conditions permit.

The region of optimum coupling is determined in the following manner. The audio frequency carrier or tone from generator is induced to the line through amplifier 38, and impressed upon solenoid as hereinabove described. This operation is preliminary to the transmission of the telecommunication signals for optimum adjustments of the transmitting and receiving apparatus. Telephonic communications between stations 10 and H are established through automatic dialing or by telephone operators. The solenoid 40 is placed close to one end of the induction coil 26. The tone or audio frequency carrier'may be heard locally by the transmitter operator through earpiece l2. The position of solenoid 40 is adjusted on box l6 with respect to induction coil 26, so that a tone of maximum intensity is heard by the operator in earpiece l2. A rapid and simple determination of the optimum coupling position is had in this manner. a The signals transmitted across the telephone lines 24-25 to the telephone receiving station I! pass through the receiver induction coil I1 within the receiver call box 23. A corresponding electromagnetic field is set up about the induction coil 21 in correspondence with the transmitted signal variations. A receiver solenoid is magnetically coupled with the induction coil 21 to pick-up the signals as transmitted from the transmitting station. The pick-up solenoid 45 is connected to the input of an audio frequency am- I .plifier 46 by input leads 41-48.

The output of amplifier 46 is connected either directly to the translator 50 or in series with a detector station 5! where the higher frequency or tone is to be removed or demodulated from the receiver signals. Where the detector 5| is employed, the signals at the input 52-53 to the translator will correspond exactly to the signals generated at the audio signal generator 34 of the transmitting station.

The solenoid 45 at the receiving station is position for a maximum signal transfer or pick-up with respect to receiver induction coil 21. This position is determined in a manner similar to that described at the transmitter, namely the axis of solenoid 5| preferably coincides with the axis of induction coil 21, and one end of solenoid 45 is juxtaposed on the metal casing 54 of the receiver phone box 23. The receiving coil 45 is experimentally adjusted to this optimum position by listening to the tone in earpiece 61 and is rigidly clamped in place as described in application Serial No. 65,869 supra. Such adjustment is preferably made during the transmission of aconstant amplitude carrier from the transmitter. The optimum; position may more accurately be indicated by an output meter (not shown) connected at the receiver audio amplifier 46 as will be evident to those skilled in the art.

Figure 2 is a graphical representation of a typical overall transmission characteristic which I have obtained between a transmission and receiving station, corresponding to the ratiobetween the signals at the generator 34 and translator 50 of Figure 1. The curve 551s plotted with frequency, in cycles per second, and with E, representing the received voltage at the translator 50. At the transmitting station, signals of constant amplitude were introduced to the audio amplifier 38 at frequencies extending from 25 cycles to 5000 cycles and beyond. Curve 55 demonstrates the substantial linearity of this method of transmission from 900 cycles to 5000 cycles as indicated by the corresponding horizontal portion of the curve 55. However, a peak 56 occurred in the region of 400 cycles; starting at 25 cycles rising to a maximum at about 400 cycles, and diminishing to the flat characteristic portion of curve 55 at about 800 or 900. cycles. With the transmission characteristic illustrated in Figure 2, an audio frequency carrier of 3000 cycles may be used to transmit telecommunication signals with no frequency discrimination for the sidebands of the 3000 cycle carrier as will be understood by those skilled in the art.

If intermediate equipment between "the telephone stations l0 and I! introduce a cut-01f frequency below 5000 cycles to the transmission characteristic, the audio frequency carrier may accordingly be adjusted so that the sidebands may be uniformly transmitted. It is also practi-' cal to transmit a single sideband upon an audio frequency carrier, and utilize the linear transmission characteristic on single side band audio frequency can'ier transmission as is well known in the art.

Another important feature of -my invention resides extending the linear characteristic transmission relation up from 25 cycles, by eliminating the efiect of the peak 56 by inserting compensation network 43 between the transmitter solenoid 40 and the transmitter amplifier 38 output. The purpose of this network is to attenuate signals between 25 cycles and 900 cycles inversely proportional to their peaked relation at 56 of Figure 2. In Figure 1, I illustrate a simple form of compensation network for this purpose. An.

inductance 51 and capacitance 58 are connected in series across solenoid 40 and'connection leads 4l-42. The inductance 51 and capacitance 50 are designed to be tuned to 400 cycles or the peak 56 frequency, in a manner well known in the art. The compensation network 43 will absorb signals to the greatest extent at the frequency f it is tuned to, namely, 400 cycles, to correspond to the peak of the induction coupling system herein described.

Figure 3 illustrates the transmission characteristic 60 of the compensation network 36. It will be seen that minimum 6| transmission occurs at 400 cycles. The proportion of transmission decrease is made equal to the proportion of the peak 56 to the plateau 55 of Figure 2.

Figure 4 represents the overall transmission characteristic 62 corresponding to Figure 2 with the compensation network 43 connected as illustrated in Figure 1. The superposition of compensation network characteristic 50 of Figure 3, upon the transmission of characteristic 55-56 of Figure 2, produces the transmission characteristic 62 of Figure 4, which is substantially linear from 25 cycles to 5000 cycles and above.

It is accordingly evident that the electrical induction method for transmitting audio frequency signals according to my present invention, may. be attained with substantially no amplitude discrimination with respect to the-frequency. I have also found that the noise'level of this transmission system is'negligible as compared to the telecommunications signal strength, resulting in an excellent signal to noise ratio.

The wide channel afforded by my present invention with no physical connection to the telephone lines may carrier wave transmission. Accordingly, in Figure 4 I have illustrated two audio frequency carriers, namely carrier No. l and carrier No. 2. Carrier No. 1 may, for example, be a tone of 1500 cycles, and carrier No. 2, a tone of 3500 cycles. The fiat frequency characteristic 6! of the system permits the side-bands of the carriers Nos. 1 and 2, representing independent or interdependent telecommunications signals, to be transmitted without any signal discrimination or interference.

The utilization of two carriers as herein described may be for simultaneous telegraphy and printing telegraphy, or telegraphy and speech, or telepicture and speech, or the simultaneous transmission of two independent similar systems, for example, two facsimile pictures being simultaneously transmitted. Another modification for utilizing thewide audio frequency channel with two independent audio frequency carriers is to simultaneously transmit adjacent lines of a picture being scanned with two scanners oppositely disposed upon the picture drum, so as to cut in half the time for transmitting a single picture. Such a system requires each of the audio frequency carriers to be associated with one of the corresponding alternate scanning lines of the picture being transmitted, to actuate corresponding alternate telepicture translators for simultaneously reproducing the predetermined alternate picture lines at the receiver.

Figure 5 is a schematic circuit diagram of a modification of my invention wherein a plurality of independent signals are transmitted over respective independent carrier frequency. The transmitter telephone station 63 is placed in operative connection with the receiver telephone station 64 over telephone line 65. Transmitter No. 1 comprises an audio frequency signal generator which generates audio frequency signals to be transmitted, and an audio frequency amplifier for amplifying these signals to a proper voltage for actuating the corresponding transmitter sole noid 65. Carrier No. 1 is introduced to the transmitter No. 1 and may, for example, be an audio frequency corresponding to carrier No. 1 shown in Figure 4, namely 1500 cycles.

Transmitter No. 2 represents a second audio frequency signal transmitter which is coupled to the telephone system through solenoid 66. Carrier No. 2 is used in conjunction with transmitter No. 2 and corresponds to carrier No. 2 of Figure 4 of a valve of 3500 cycles. The signals to be independently and simultaneously transmitted from transmitters Nos. 1 and 2 over telephone system 63-64 are respectively coupled to the phone box 51 of the transmitter station 63 through solenoids 65 and 66. Their respective carriers Nos. 1 and 2 are sufficiently separated so that the side bands of the corresponding modulated signals do not interfere. The solenoids 65 and 66 are mounted upon the phone box 61 in optimum coupling relation with the induction coil 68 contained therein. A common yoke 69 is indicated in dotted lines for properly supporting solenoids 65 and 65 upon phone box 61.

A compensation network is connected at the output of the transmitter which transmits frcquencies falling within the region of the peak portion 58 of the transmission characteristic as indicated in Figure 2. In this illustration of Figure be utilized with more than one mitted through filter No.

signals generated at transmitters 5, the compensation network is connected in the circuit of transmitter No. 1 since it utilizes the lower carrier frequency. A filter circuit is connected between solenoids 65 and 86 and their respective transmitters in order to insure separation of the side bands between the two modulated carrier signals as will be evident to those skilled in the art. I

At the phone box I0 of the receiving station 64, solenoids Ii and 12 are arranged on opposite sides of the induction coil 13. The received signals generate corresponding electromagnetic waves which are picked-up by the solenoids H and I2. Solenoid 1| is coupled to a corresponding translator No. 1; and solenoid 12, to translator No.2. Filter No. 1' is inserted between solenoid H and the translator No. 1 and is designed to pass frequencies corresponding to those trans- 1 at the transmitter. In a similar manner, filter No. 2 is insertedbetween solenoid l2 and translator No. 2 to insure the reception at translator No. 2 of the frequency band transmitted across filter No. 2. A common supporter yoke 14 is preferably used for the solenoids II and transmitting station.

The translators Nos. 1 and 2 correspond to independent receiver, stations embodying audio frequency amplifier 46, detector 5| and translator 50, as illustrated in Figure 1. The duplex transmission system employing two independent carriers accordingly transmits audio frequency Nos. 1 and 2.

The duplex transmission system employing two independent carriers, accordingly transmits over the telephone system "-44, audio frequency signals generate at transmitters Nos. 1 and 2 which modulate respective independent carrier frequencies, the corresponding translators Nos 1 and 2 receiving their respective signals and independently translating them as will now be evident to those skilled in the art.

The telecommunication system illustrated in Figure 5 may be employed in tions. Transmitter No. 1 may, for example, be a printing telegraph system whilst transmitter No. 2 may be used to transmit picture simultaneously. .Translator No. 1 accordingly would translate the messages of the printing telegraph, while translator No. 2 reproduces the transmitted pictures. A similar modification would result when the telepicture signals are transmitted over the higher carrier, 3500 or 4000 cycles corresponding to transmitter No. 2 of Figure 5, and voice communication is impressed across solenoid 65 through filter No. 1, filter No. 1 being designed to cut-off the frequency above 2000 cycles so that they may not interfere with the frequency band of transmitter No. 2. When speech is employed, his not necessary to employ a carrier No. 1 frequency and the frequency range between 25 and 2000 cycles may be directly transmitted and correspondingly received through filter No. 1 at the translator No. 1. In this case, transmitter No. 1 utilizes a microphone while translator No. 1 employs an earpiece.

A single solenoid is employed for inductively coupling a plurality of independent units to the telephone system. Referring to Figure 6, cperative connection is established between telephone stations 15 and 18 over the telephone line 11. The transmitter telephone station'phone box 18 is coupled with the transmitting solenoid 19' in a manner similar to the hereinabove described modification.

12 similar to the yoke 69 at the several modificaaua'rw to the transmitter solenoid 19 which is placed in-..

optimum coupling relationship with the induction coil 85 within phone box 18. g

It is to be understood that the transmitters A, B and C comprise their respective audio frequency signal generator and audio frequency amplifier together with independent carrier frequencies. The carrier frequencies for transmitters A, B and C should be spaced along the audio frequency spectrum so that their respective side bands due to the modulating signals do not interfere, as will be understood by those skilled in the art. i

In order to provide a substantially linear transmission frequency characteristic for the system as a whole, the transmitters A, B and C which generate signals lying within the band, which is normally non-linear, comprise proper compensating networks, for example, corresponding to network 43 of Figure 1, in order to effect linear transmission of the respective transmitter sig male.

The signals from primary windings ill, 82 and 83 of transformer Bil are accordingly spaced so as to be non-interfering, and their effect upon the secondary winding 84 is the super-position of the plurality of modulated carriers. The plurality of modulated carrier frequency waves are induced across the telephone system 15-48 through the solenoid 79 as will now be understood.

The signals induced across the telephone line H to the receiving station "pass through the induction coil 86 located within the receiver phone.

box 81 and produces electromagneticwaves corresponding to the transmitted signals. A receiver solenoid 88 placed in optimum coupled relationship with the induction coil 86 picks-up the signals and conducts them to the primary 89 of the receiver transformer 90. Transformer 9d contains three secondary windings 9|, 92 and 93 which are respectively connected to translating units A, Band C through corresponding filter units A, B and C. I

The voltage induced to the secondary windings of transformer 90 by primary winding 89 corresponds to voltage at the output winding 84 of transmitter transformer 80. This voltage is'ln-= duced across all the secondary windings 9f, 92 and 93. However, by providing suitable filters in the circuits between the secondary windings to the translator units, suitable segregation of the combined signal voltages is had. For example,.fllter unit A, connected to the input of translator A,

is designed having a transmission frequency characteristic encompassing the band of frequencies at the output of transmitter A, and substantially cutting-off signals outside this band width. In a similar manner, filter B, connected to the input of translator B, is designed to transmit only the frequency band from transmitter B; and filter C. connected to translator C, to transmit the band of transmitter C.

It will now be evident that a plurality of modulated carrier wave signal bands lying within the transmission frequency range over the telephone system 15-16, may be transmitted without physical interconnection with the telephone system,

but by electromagnetic induction at the transmitter and receiver. It is to be understood that a greater number of independent transmitter units may be used over the frequency channel respective translator unit.

which, for example, may extend from 25 cycles to 5000 or even 7000 cycles, by employing single sideband transmission of the signals. As is known to "those skilled in the art, a modulated carrier wave normally comprises two sets of side-bands on either side of the carrier frequency. By suppressa ing one of the side-bands, practical transmission of the audio frequency signals may be accomplished with a single side-band. The respective filter units at the receiving station are designed to pass a predetermined frequency band corresponding to the signals to be introduced to the The transmitter units schematically illustrated as A, B and C, may be any type of audio frequency signal generator such as a telepicture set for transmitting pictures or written messages, a telegraph set, a printing telegraph unit, a speech or microphone unit, etc. Although I have illustrated three units, A, B and C, it will be obvious-that any number of such units may be used simultaneously in this modification as long as their frequency bands are properly chosen over the 1 transmission'characterlstic so as to be non-interfering.

secondary 84 of the coupling transformer 30 and l the induction solenoid 19. The primaries ill, at and 83 of the coupling transformer 80 in this modification, will be connected to the outputs of signal generator units corresponding to units A, B and 0, these signal generating units comprising the respective carrier frequency sources and the independent audio signals to be transmitted. A common amplifier unit is accordingly more economical and compact for the plurality of signal bands. In a similar manner, the amplifier units associated with translator units A, B and C at the receiving station may be combined into a common amplifying unit inserted at the output of receiving solenoid 88 connected between solenoid .88 and primary winding 80 oi coupling transformer 90. The filter units A, E and C and the associated translator units A, B and C in this modification are respectively connected to the secondary windings M, M and 9d of coupling transformer 90.

Another modification following along the lines of the modification illustrated in Figure 6, resides in a novel high speed printing telegraph system employing individual tones or frequencies. The five code combinations, for example, Baudot code combinations, of the characters to betransmitted may be associated with an individual frequency. The five frequencies are sumciently spaced along the frequency transmission channel to avoid in under the control of the combined signal effect during each character transmission.

Although I have described preferred embodiments for carrying out my present invention, it will be evident to those skilled in the art that modifications falling within the spirit and scope of my invention are feasible and accordingly I do not intend to be limited except as set forth in the following claims.

I claim: Y

1. In a signalling system over a telephone line having a telephone station at each end and a phone box including an induction coil associated with each station: means for producing a first audio frequency current band in the range of 1500 cycles per second; means for producing an independent second audio frequency current band in the range of 3500 cycles per second; and means at one telephone station for transmitting said first and second independent audio frequency current bands across said telephone line comprising a first and second solenoid operatively connected respec-' tively to said first and second audio frequency band currents, said solenoids being electromagnetically coupled with said induction coil associated with said first mentioned station; a common U-shaped yoke for supporting said solenoids on opposite sides of the associated phone box in optimum coupling relation with said induction coil and compensation means connected in said first audio frequency current circuit designed to transform the lower frequencies to provide a substantially linear transmission frequency characteristic for said transmitted bands.

2. In a signalling system over a telephone line having a telephone station at each end and a phone box including an with each station: means for producing a first audio frequency current band in the range of 1500 cycles per second; means for producing an independent second audio frequency current band in the range of 3500 cycles per second; means at one telephone station for transmitting said first and second independent audio frequency current bands across said telephone line, comprising a first and second solenoid operatively connected respectively to said first and second audio frequency band currents, said solenoid being electromagnetically coupled with said induction coil associated with said first mentioned station, a com mon U-shaped yoke for supporting said solenoids on opposite sides of the associated phone box in optimum coupling relation with said induction coil, and compensation means connected in said audio frequency current circuits for providing a substantially linear transmission frequency characteristic for saidtransmitted bands; means at the second telephone station for receiving said transmitted frequency bands comprising a third and fourth solenoid electromagnetically coupled with the induction coil associated with said second station and a second U-shaped yoke for supporting said third and fourth solenoids on opposite sides of said second station phone box in optimum coupling relation with the associated induction induction coil associated coil; and means for translating said first and second frequency bands connected respectively to said third and fourth solenoids.

3. In a signalling system over a telephone line having a telephone station at each end and a phone box including an induction coil associated with each station: meansfor producing a first audio frequency current band to be transmitted comprising a first carrier frequency current source of the order of 1500 cycles per second, means for modulating said first carrier current in accordance with first signals to be transmitted, and means for amplifying the first modulated carrier current; means for-producing an independent second audio frequencycurrent band to be transmitted comprising a second carrier frequency current source of the order of 3500 cycles per second, means for modulating said second carrier current in accordance with second signals to be transmitted, and means for amplifying the second modulated carrier currents; and means at one telephone station for transmitting said first and second independent audio frequency current bands across said telephone line comprising a first and second solenoid operatively connected respectively to said first and second audio frequency band currents, said solenoids being electromagnetically coupled with said induction coil associated with said first mentioned station, a common U-shaped yoke for supporting said solenoids on opposite sides of the associated phone box in optimum coupling relation with said induction. coil, and compensation means connected in said first audio frequency current circuit for providing a substantially linear transmission frequency characteristic for said transmitted bands.

4. In a signalling system over a telephone line having a telephone station at each end and a phone box including an induction coil associated with each station: a plurality of independent generators of signals to be transmitted; a carrier frequency current source associated with each of said signal generator, said carriers being spaced within the 5000 cycle band of the telephone system; means for modulating said carrier frequency currents by said corresponding signals to be transmitted for producing a plurality of independent modulated carrier frequency bands; means at one telephone station for transmitting said plurality of frequency bands across the telephone line comprising a solenoid electromagnetically coupled with the induction coil associated with said first mentioned station and a compen-. sation network connected in the transmitting circuit for providing a substantially linear transmission frequency characteristic for the transmitted bands across the telephone circuit; and means at the second telephone station for receiving and translating said plurality of frequency bands comprising a solenoid electromagnetically coupled with the induction coil associated with said second station, and a plurality of translating units individual to each of said frequency bands.

WILLIAM G. H. FINCH. 

